With the development of information technologies, submarine cable networks cover major sea areas, and the submarine cable network has become a main communications network that bears international communications services. A submarine cable communications system for implementing the submarine cable network achieves, by using an optical add-drop multiplexer (Optical add-drop Multiplexer, OADM for short), service communication between multiple connected sites, and also features a high price/performance ratio, a short delay, and a high availability. However, in the prior art, however, an optical add-drop multiplexer apparatus that uses an OADM branch device is inflexible in routing, and if a bandwidth needs to be changed, the optical add-drop multiplexer apparatus needs to be taken out of water to replace the OADM branch device. A reconfigurable optical add-drop multiplexer (Reconfigurable Optical add-drop Multiplexer, ROADM for short) is designed in the industry regarding this problem, so that a requirement for an adjustable bandwidth of an add/drop wave is met and flexible routing is implemented.
However, in the prior art, when a trunk transmission line or a branch transmission line of a line fails, a failure detection path provided by the ROADM is a reverse path of a detection signal input path. The ROADM has multiple reconfiguration states; therefore, in a case in which the reconfiguration state of the ROADM cannot be acquired, a transmission path of a returned signal cannot be learned, that is, it cannot be determined whether a trunk transmission line or a branch transmission line of the line fails. In addition, after the reconfiguration state of the ROADM is set, a returned signal acquired at a signal input port can only be used for failure detection of one transmission path, which causes inconvenience to line failure detection.